Currently the DO firewall filters IP Protocol 47 (GRE) packets, which interferes with pptp vpn links. If GRE packets were added to the list, along with ICMP, IPv4 and IPv6, this would allow the use of PPTP VPNs (see e.g. https://www.digitalocean.com/community/tutorials/how-to-setup-your-own-vpn-with-pptp) and other VPNs that utilize GRE packets, such as IPSec